ok so much of this is in the upcoming update, we made a snap decision to go to taiwan because of the 30 limit on the china dual-entry visas. i'm staying with a friend who has a stack of equipment including IR oven, small pick-and-place machine, scope etc. so it's a much easier arrangement, i'm helping him out as well.
the rk3288 board is going to first prototyping, we're getting quotations from PCB manufacturers. that leaves me free to move to the next PCB design, and i picked the passthrough card... which turns out to be much more complicated than i was expecting. i _could_ do a simple version (just a TFP401a) but it would have to have 2 USB ports and a break-out arrangement for the remaining pins.... i decided instead to wire them all to a STM32F072.
now, the idea here is to pre-load a userspace USB-based DFU bootloader into the NAND flash, and to require firmware to be uploaded after it reports the contents of the EOMA68 EEPROM ID. the TFP401a's I2C will be connected to the STM32F072, it can emulate an EEPROM and can load EDID data which tells the TFP401a what the LCD's resolution is.
normally the LCD resolution would be fixed... but here it is not possible to know what the LCD resolution is going to be.... also we have no idea in advance what the various functions are of the EOMA68 pins: maybe the SD/MMC pins are used for an SD card (in which case the STM32F072 can be programmed to be a USB mass-storage device), but maybe they are used as GPIO, or maybe they are used for a WIFI SIP module: we just don't know. hence the need to be flexible and for firmware to be uploaded on a per-boot basis.
i would _like_ to use a more powerful EC but i haven't the time to learn a new EC... so the STM32F072 it is.
l.
--- crowd-funded eco-conscious hardware: https://www.crowdsupply.com/eoma68